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Using the standard sign convention (light travels left to right, distances measured from pole, +ve to right), the MIRROR FORMULA relating object distance $u$, image distance $v$, and focal length $f$ is
A$1/v - 1/u = 1/f$
B$1/v + 1/u = 1/f$
C$1/u + 1/f = 1/v$
D$v + u = f$
Answer & Solution
Correct answer: B. $1/v + 1/u = 1/f$
1. NCERT §9.2.5 derives the mirror formula from the geometry of paraxial rays reflecting off a spherical mirror.
2. The result: $\dfrac{1}{v} + \dfrac{1}{u} = \dfrac{1}{f}$.
3. Sign convention (Cartesian): u, v, f are all NEGATIVE for real objects, real images and concave mirrors (since they're left of the pole).
4. Distinct from the LENS formula $1/v - 1/u = 1/f$ (different sign — option A is the lens formula, not the mirror one).
5. Numerical example: concave mirror $f = -10$, object at $u = -30$ → $1/v = 1/-10 - 1/-30 = -1/15$, so $v = -15$. Image at $15\,\text{cm}$ in front of mirror.
6. Option C is rearranged but logically equivalent (sign-wise also fine); however the canonical NCERT form is option B. Option D has wrong functional form.
_Source: NCERT Class 12 Physics Part 2, Ch 9, §9.2.5 (Mirror Formula, Eq. 9.4), p. 4–5._
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